The invention relates to a method and an installation designed to carry out a three phase chemical reaction under pressure, such as a reaction in which a liquid product is reduced by a gaseous reducing agent, in the presence of a solid catalyst.
The invention can be applied to all cases in which a chemical reaction is carried out in the presence of a gas phase and two non-gaseous phases, at least one of which is a liquid. Depending on the case, the other non-gaseous phase is either a solid or a liquid.
A favored application relates to the reaction of reducing uranyl nitrate to uranous nitrate, in which the reducing agent is hydrogen under pressure and the catalyst is made up of platinum on a silica carrier.
Three phase reactions, such as reduction reactions which are carried out in the presence of a catalyst are carried out in chemical reactors. In most cases, reactors with a stationary bed, reactors with mechanical agitation or fluidized bed reactors, are used.
In stationary bed reactors, the liquid product to be treated and the gaseous reducing agent circulate in co-current or counter-current upwards inside the reactor, passing through the solid catalyst, this being retained in a fixed fashion at a defined level in the reactor.
In reactors with mechanical agitation, as for example fluidized bed reactors, the solid catalyst is in suspension within the liquid phase, inside the reactor.
In the case of reactors with mechanical agitation, the suspension of the solid catalyst is provided by mechanical agitation of the different phases present inside the reactor.
In the case of fluidized bed reactors, it is the displacement of the fluid (liquid or gaseous) which, above a certain speed threshold, brings about the suspension of the solid catalyst.
Certain three phase chemical reactions are characterized by the particular operating conditions such as the presence of a gas phase under pressure and the strongly exothermic nature of the reaction. The reduction of uranyl nitrate to uranous nitrate belongs to this category. Hence, the pressure of hydrogen is preferably between about 30 bars and about 60 bars. Furthermore, the reaction produces 33 kcal/mole of converted uranium, while it is desirable to be preferably between about 10xc2x0 C. and about 20xc2x0 C. in order to avoid parasitic reactions.
Currently, this type of reaction using a gaseous reducing agent under pressure is carried out industrially in stationary bed reactors. However, this technology has a certain number of disadvantages.
Hence, stationary bed reactors have a large pressure drop, because the liquid has to pass through the catalyst bed and the sintered element. This phenomenon which requires the catalyst to be subjected to high pressure, encourages the active part of the catalyst to be detached from its carrier by attrition. From this stem the risks of corrosion of the welds of the reactor body and of the sintered element, which also risks being blocked by the fine detached particles. So as to prevent these problems, it is necessary to change the sintered element retaining the catalyst regularly (for example every 300 hours) as well as the reactor body (for example, once a year).
In addition, the operation of such a reactor means having a temperature gradient between the hot upper part and the cold lower part. The control of the temperature in the center, which is necessary to prevent the parasitic reactions when the reduction reaction is strongly exothermic, means that a relatively large number of temperature sensors (for example, five) has to be used. In effect, the presence of hot spots (common in stationary bed reactors), associated with differences in reactivity, is difficult to detect.
Apart from this, in stationary bed reactors, the fall-off in catalyst activity requires regular replacement of the catalyst (for example, once a year).
If the products being processed are dangerous or radioactive, the replacement of movable components in contact with these products requires complex intervention operations in order to guarantee safety.
Among other reactors currently used to carry out three phase reactions, mechanical agitation reactors provide very good contact between the phases. However, they encourage attrition of the solid catalyst. Furthermore, they have the disadvantages of high energy consumption and problems associated with providing seals due to the presence of moving components that pass through the walls.
In addition, fluidized bed reactors remain generally restricted with regard to material and heat transfer, which can pose delicate problems in the case of a reaction that is strongly exothermic. Furthermore, the input flow rate of the liquid phase is restricted to relatively low values.
In addition, gas siphon type reactors are known which comprise a central region called the riser and an annular region called the downcomer, separated by a cylindrical partition. In these reactors, a liquid recirculation loop is created by injecting a gas into the lower part of this central region, in a way that provides an ascending circulation in this central region and a descending circulation in the annular region.
Currently, reactors of this type are only used industrially in very well defined fields which are the treatment of waste water, aerobic fermentation (agro-food field) and the growth of micro-organisms (pharmaceutical field). This equipment operates at atmospheric pressure or under moderate pressure (less than 5 bars).
The precise subject of the invention is a method and an installation designed to carry out a three phase reaction under pressure while avoiding all the disadvantages of the techniques which have been used up to now for this purpose, in particular requiring reduced maintenance, permitting simplified control of the reaction and allowing charging, discharging and regeneration of the catalyst to be automated.
To this end, a method of carrying out a three phase chemical reaction under pressure is proposed that involves a gas phase and two non-gaseous phases, at least one of which is liquid, characterized in that it comprises the following steps:
circulating, in a closed loop and co-currently, the two non-gaseous phases, in a reactor, by the injection of the gas phase into the bottom of a central region of the reactor, in a way that creates an ascending circulation in said central region and a descending circulation in the annular region of the reactor, separated from the central region by a cylindrical partition
separation and recovery, in an upper region of the reactor, of the excess gas phase and a liquid fraction corresponding to the input flow rate of the liquid phase;
separate routing of the excess gas phase and the liquid fraction into a separator, which may include a high pressure separator, outside the reactor; and
adjustment of the pressure in the reactor and the level in the separator, by adjustment of gas flow rate and the liquid flow rate leaving the separator.
The use of a gas siphon type of reactor connected to a separator, such as a high pressure separator, enables one to regulate the pressure and the level in the reactor in a simple way, while benefitting from the advantages provided by this type of reactor.
Preferably, the liquid fraction is recovered in the upper region of the reactor, through a lateral branch pipe positioned behind a profiled wall that inflects the circulation to the annular region of the reactor and the liquid fraction is filtered from any possible traces of solid material, such as catalyst, carried along to the branch pipe inlet.
According to the reaction used, the gaseous and non-gaseous phases are cooled and heated inside the reactor.
In a preferred application of the invention, a reduction reaction is carried out under pressure, of a liquid product by a gaseous reducing agent, in the presence of a solid catalyst.
Preferably, in this preferred application of the invention, the solid catalyst is periodically regenerated inside the reactor, by carrying out the following steps:
discharge of the liquid phase;
filling the reactor with water;
sparging with an inert gas, for a specified time;
emptying the water.
The invention particularly relates to the reduction of uranyl nitrate with hydrogen in the presence of platinum on a silica carrier.
Another subject of the invention is an installation for carrying out a three phase chemical reaction under pressure, that involves a gas phase and two non-gaseous phases, at least one of which is liquid, characterized in that it comprises:
a reactor including a central region and an annular region, separated by a cylindrical partition, means for injecting the gas phase into the bottom of the central region, to create closed loop and co-current circulation of the two non-gaseous phase, ascending in the central region and descending in the annular region; said reactor further including an upper region for the separation and recovery of the excess gas phase and a liquid fraction that corresponds to the input flow rate of the liquid phase;
a separator, which may include a high pressure separator, outside the reactor and connected to the upper region of the reactor, so as to separately route the excess gas phase and the liquid fraction into the separator, and
means of adjusting the gas flow rate and the liquid flow rate leaving the separator, so as to regulate the pressure in the reactor and the level in the separator.